Electrodynamic transducers or electrodynamic speakers, respectively, are used in many, particularly mobile appliances, like for instance portable radios or playback equipment as e.g. mp3-players, small disk players or pocket sized tape players but also in mobile telecommunication terminals like e.g. mobile phones, personal digital assistants or the like for an implementation of a variety of acoustic functions like for instance voice or audio data transmission or a generation of certain signalling tones. As mobile appliances are continuously subject to miniaturisation, the available space left for integrating a speaker is also shrinking continuously. Acoustic engineers will therefore seek for electrodynamic transducers or speakers, respectively, with continuously smaller membrane area and/or reduced thickness.
For most small loudspeakers, the circumference of the membrane is significantly smaller than the wavelength of the sound produced by the speaker at least in the lower audio frequency range. To compensate for the resulting impairment of the transducing efficiency at lower frequencies, the membrane velocity has to be increased inversely proportional with decreasing frequency. A uniform transducing efficiency can thus be obtained within the complete frequency range of a respective electrodynamic loudspeaker.
To this effect, the membrane resonant frequency of an electrodynamic transducer is set close to the lower end of the frequency range to be transduced, and the electrodynamic loudspeaker is operated with a constant voltage drop across its coil impedance. The latter requirement can easily be accomplished by operating the speaker with a low output impedance audio circuit. The combined inertial mass of the membrane, the coil of the speaker, and the moved air will thus be driven with a more or less constant force.
When being operated with a frequency independent constant voltage drop, the oscillation amplitude of the electrodynamic transducer membrane is inversely proportional to the square of the signal frequency. High membrane oscillation amplitudes at low frequencies have to be expected for this operation mode. If the oscillation amplitude of the membrane exceeds a certain limit, an electrical audio frequency signal will not be transduced properly into a respective sound signal, but distortions and clipping will occur. Sometimes, the speaker would even be damaged. The level allowed for audio frequency signals at low frequencies is therefore limited by the maximum oscillation amplitude of the speaker membrane. It is to be noted, that the explained applies particularly also to mobile telecommunication terminals with leak tolerant as well as to those with a not leak tolerant design of an acoustic cabinet for an electrodynamic transducer. Where applicable in the further description, the abbreviation ‘audio signal’ will be used for a denomination of an audio frequency signal usually in an electrical form.
The overall efficiency of an electrodynamic transducer is strongly correlated with the size of its membrane area. Larger membrane areas yield a higher efficiency than smaller ones. To improve the efficiency of a given electrodynamic speaker with a respectively small membrane area, the membrane and its suspension have to be engineered for a scaled-up oscillation amplitude. The usual requirement of designing an electrodynamic speaker as thin as possible sets a certain limit to respective development trends. For a given signal with a given spectrum, the sound pressure achievable with a thin and small electrodynamic transducer is therefore limited by the small range allowed for membrane oscillation amplitudes.
Improvements of the transducing efficiency can be achieved by reducing the inertial mass of a speakers moving parts which are formed by the membrane, the driving coil, and the suspension. As the stiffness of the membrane suspension cannot be reduced below a certain limit, speakers designed for a high dynamic range, i.e. a high range in sound volume, usually have an exceptionally high membrane resonant frequency thus raising the lower limit frequency of the transducing frequency response.
An acoustic engineer seeking for a good sound quality and a high dynamic range for an acoustic speaker of a small acoustic system like e.g. that of a mobile telecommunication device therefore has to find a compromise between lowering the resonant frequency for an improvement of the sound quality and raising the resonant frequency for an extended dynamic range.
EP 0 071 845 B1 proposes an audio system for a hearing aid which splits the audio signal into several channels, amplifies the signal of each channel individually, and combines the thus processed individual signals for being transduced by a small loudspeaker. The improvement in sound quality achieved is obtained by a high complexity of the circuitry as separate filters and amplifiers are required for each channel.
The same drawback has to be stated for the system proposed in WO 94/23 548. In addition to EP 0 071 845 B1, the system defines two thresholds for the output signal of each channel, the gain of which is increased for the output signal falling below the lower threshold value defined and reduced for the output signal exceeding the higher threshold value defined.
U.S. Pat. No. 4,739,514 and U.S. Pat. No. 5,361,381 superimpose to an original audio signal an amplified lowpass filtered signal of the same origin to boost the bass frequency range of the audio signal. The relative boost of the bass frequency range varies with the strength of the audio signal. It is high for low signal levels and low for high signal levels for to compensate for the low sensitivity of the human hearing for bass frequencies at low sound pressures. The proposed system is designed for high power speaker systems which use membranes of sufficient size to transduce audio signals with high levels in the lower frequency region. U.S. Pat. No. 5,361,381 applies the said to the woofer of a loudspeaker system only.